Database indexes: what are they for, why can't they be applied to everything, how to analyze slow queries?

Indexes in a database are needed to speed up search and data retrieval. They create a structure (for example, a B-tree) that allows quick access to the required records without scanning the entire table.

Why can't we create indexes on everything:

• Indexes occupy additional space on disk.
• When inserting, updating, or deleting data, indexes need to be updated, which slows down these operations.
• Excessive indexes can degrade performance and complicate query plans.

Analysis of slow queries:

• Use profilers and slow query logs (for example, slow query log in MySQL).
• Look at execution plans (EXPLAIN) to understand which indexes are used.
• Identify queries that cause delays most often and optimize them by adding necessary indexes or rewriting queries.

Example of EXPLAIN in PostgreSQL:

EXPLAIN ANALYZE SELECT * FROM users WHERE email = 'example@example.com';

This will show whether an index on the email field is used and how long the query takes.